Power converters

ABSTRACT

The present invention provides_a power converter that can be used to interface a generator ( 4 ) that provides variable voltage at variable frequency to a supply network operating at nominally fixed voltage and nominally fixed frequency and including features that allow the power converter to remain connected to the supply network and retain control during supply network fault and transient conditions. The power converter includes a generator bridge ( 10 ) electrically connected to the stator of the generator ( 4 ) and a network bridge ( 14 ). A dc link ( 12 ) is connected between the generator bridge ( 10 ) and the network bridge ( 14 ). A filter ( 16 ) having network terminals is connected between the network bridge ( 14 ) and the supply network. A first controller ( 18 ) is provided for controlling the operation of the semiconductor power switching devices of the generator bridge ( 14 ). Similarly, a second controller ( 46 ) is provided for controlling the operation of the semiconductor power switching devices of the network bridge ( 14 ). The first controller ( 18 ) uses a dc link voltage demand signal (VDC 13  GEN*) indicative of a desired dc link voltage to control the semiconductor power switching devices of the network bridge ( 10 ) to achieve the desired level of dc link voltage that corresponds to the dc link voltage demand signal (VDC 13  GEN*). The second controller ( 46 ) uses a power demand signal (P*) indicative of the level of power to be transferred from the dc link to the supply network through the network bridge ( 14 ), and a voltage demand signal (VTURB*) indicative of the voltage to be achieved at the network terminals of the filter ( 16 ) to control the semiconductor power switching devices of the network bridge ( 14 ) to achieve the desired levels of power and voltage that correspond to the power and voltage demand signals (P* and VTURB*).

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of United Kingdom Patent ApplicationNo. 0523087.5, filed Nov. 11, 2005, U.S. Patent Provisional ApplicationSer. No. 60/736,205, filed Nov. 14, 2005 which was converted to U.S.patent application Ser. No. 11/292,669, filed Dec. 2, 2005.

FIELD OF THE INVENTION

The present invention relates to power converters, and in particular topower converters that can be used to interface generators providingvariable voltage at variable frequency to a power grid or supply networkat nominally fixed voltage and frequency. The present invention alsoincludes features that allow the power converters to remain connected tothe supply network and retain control during network fault and transientconditions. The power converters are particularly suitable for use with,but not restricted to, generators that are driven by wind turbines.

BACKGROUND OF THE INVENTION

It is possible to convert wind energy to electrical energy by using awind turbine to drive the rotor of a generator, either directly or bymeans of a gearbox. The ac frequency that is developed at the statorterminals of the generator (the “stator voltage”) is directlyproportional to the speed of rotation of the rotor. The voltage at thegenerator terminals also varies as a function of speed and, depending onthe particular type of generator, on the flux level. For optimum energycapture, the speed of rotation of the output shaft of the wind turbinewill vary according to the speed of the wind driving the turbine blades.To limit the energy capture at high wind speeds, the speed of rotationof the output shaft is controlled by altering the pitch of the turbineblades. Matching of the variable voltage and frequency of the generatorto the nominally constant voltage and frequency of the power network canbe achieved by using a power converter.

U.S. Pat. No. 5,083,039 describes a variable speed wind turbine wherethe rotating shaft of the wind turbine is used to drive the rotor of anac induction generator. A power converter is used to interface thegenerator output to a power network. The power converter includes activesemiconductor power switching devices that control the stator electricalquantities in each phase of the generator. A torque command device isused to derive a torque demand signal indicative of a desired torque. Agenerator controller operates under field orientation control and isresponsive to the torque demand signal to define a desired quadratureaxis current that represents torque in rotating field coordinates normalto the rotor flux field. The active semiconductor power switchingdevices are then controlled by the generator controller using a pulsewidth modulation circuit to produce stator electrical quantities thatcorrespond to the desired quadrature axis current. An invertercontroller regulates the output current to supply multi-phase ac powerhaving leading or lagging currents at an angle specified by a powerfactor control signal. In this arrangement, a loss of network voltageduring a supply dip leads to loss of control of the dc link voltage.Consequently, the ability to control the reactive current that isessential for voltage support functions demanded by the network codes isalso lost.

U.S. Pat. No. 5,225,712 expands on the principle set out above toinclude reactive power control or power factor angle control as afunction of a mode switch. In a similar manner, the inverter bridgecontroller scheme of U.S. Pat. No. 5,225,712 is solely responsible forregulating the dc link voltage. Both schemes therefore suffer from thedisadvantage that during the situation where the network voltage islost, then the dc link voltage control and the ability to controlreactive current during the voltage dip are also lost.

SUMMARY OF THE INVENTION

The present invention aims to at least reduce the above problems anddisadvantages by providing a power converter that can be used tointerface a generator that provides variable voltage at variablefrequency to a supply network operating at nominally fixed voltage andnominally fixed frequency, the power converter comprising:

-   -   a first active rectifier/inverter electrically connected to the        stator of the generator and including a plurality of        semiconductor power switching devices;    -   a second active rectifier/inverter including a plurality of        semiconductor power switching devices;    -   a dc link connected between the first active rectifier/inverter        and the second active rectifier/inverter;    -   a filter connected between the second active rectifier/inverter        and the supply network, the filter including network terminals;    -   a first controller for the first active rectifier/inverter; and    -   a second controller for the second active rectifier/inverter;    -   wherein the first controller uses a dc link voltage demand        signal indicative of a desired dc link voltage to control the        semiconductor power switching devices of the first active        rectifier/inverter to achieve the desired level of dc link        voltage that corresponds to the dc link voltage demand signal;        and    -   wherein the second controller uses a power demand signal        indicative of the level of power to be transferred from the dc        link to the supply network through the second active        rectifier/inverter, and a voltage demand signal indicative of        the voltage to be achieved at the network terminals of the        filter to control the semiconductor power switching devices of        the second active rectifier/inverter to achieve the desired        levels of power and voltage that correspond to the power and        voltage demand signals.

The power converter can be used to interface the generator to the supplynetwork during normal operating conditions but also includes featuresthat allow it to operate in situations where the supply network voltageis varying due to grid faults or transients on the supply network. Moreparticularly, the second controller can use a measure of the supplynetwork voltage to determine limits on the power that can be exportedfrom the second active rectifier/inverter when the supply networkvoltage deviates from its nominal condition and can also use the measureof the supply network voltage to determine the level of current that isto be provided from the second active rectifier/inverter to providevoltage support to the supply network when the supply network voltagedeviates from its nominal condition.

The generator can be a linear or rotating generator of any suitabletype. Examples would include an induction generator or a synchronousgenerator excited by any suitable means such as permanent magnets orconventional or superconducting field windings. In the case of arotating generator, the rotor can be connected to, or driven by, theoutput shaft of a turbine or prime mover such as a wind turbine, a tidalturbine, a hydro-turbine, a steam turbine engine, a diesel engine or agas turbine engine, for example. A linear generator could be used inapplications that inherently benefit from reciprocating motion, forexample wave power generators.

The first controller preferably uses a flux demand signal indicative ofa desired level of flux to be achieved in the generator and convertsthis to a direct axis current demand signal for the first activerectifier/inverter. The first controller can then use the direct axiscurrent demand signal to control the operation of the semiconductorpower switching devices of the first active rectifier/inverter toproduce stator electrical quantities that achieve the desired directaxis current for the first active rectifier/inverter. The term “statorelectrical quantities” is used herein to refer to any and all of theindividual phase voltage magnitude, individual phase current magnitude,phase and frequency in a multi-phase generator.

The operation of the semiconductor power switching devices in the firstactive rectifier/inverter can be controlled using gate drive controlsignals derived in accordance with a conventional pulse width modulationstrategy. It will be readily appreciated that various types of pulsewidth modulation strategy can be considered. In a preferred aspect ofthe present invention with a two-level voltage source inverter then afixed frequency pulse width modulation strategy can be implemented asfollows. In a digital processor, the output voltage requirementdetermined from the combination of direct and quadrature axis voltagesignals is multiplied by the value of a triplen enhanced sine waveformdetermined by the value of the angle at which the output voltage is tobe applied for a given phase in the active rectifier/inverter. A triplenenhanced sine waveform is used to maximise the output voltage that canbe achieved at the ac terminals of the active rectifier/inverter blockfor a given dc link voltage. The resultant signal is compared to atriangular waveform running at fixed frequency to determine the specificswitching times of the upper and lower semiconductor power switchingdevices in that phase of the active rectifier/inverter. To overcome theknown switching delays and prevent the simultaneous conduction of theupper and lower semiconductor power switching devices, blanking periodscan be imposed on the specific switching times between the uppersemiconductor power switching device being turned off and the lowersemiconductor power switching device being turned on. Correspondingblanking periods are imposed between the lower semiconductor powerswitching device being turned off and the upper semiconductor powerswitching device being turned on. The same process is repeated for eachphase of the active rectifier/inverter.

The first controller preferably converts the flux demand signal to thedirect current axis demand signal with reference to one or morecharacteristics of the generator. The characteristics might include thegenerator equivalent circuit parameters and/or nameplate data such asrated current, voltage, speed, power and frequency and data such as themagnetisation curve. The magnetisation curve provides the relationshipbetween stator flux for the generator and the direct axis currentnecessary to achieve it. Typically, the magnetisation curve for agenerator will show a linear relationship between stator flux and directaxis current up to a certain level of flux. However, beyond this certainlevel small increases in flux will require larger increases in directaxis current. This non-linear region is associated with the saturationof the iron used to form the magnetic circuit of the generator. Themagnetisation curve can be derived from the test process for thegenerator during its manufacture or by current injection tests carriedout during the generator commissioning step. Such current injectiontests can be arranged to be automatic as part of a self-commissioningroutine for the power converter/generator combination.

The first controller preferably compares the dc link voltage demandsignal indicative of a desired dc link voltage to a dc link voltagefeedback signal to determine a quadrature axis current demand signal forthe first active rectifier/inverter. The first controller can thencontrol the semiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired quadrature axis current for the first activerectifier/inverter.

The second controller may supply a control signal that varies inaccordance with the prevailing supply network voltage conditions to thefirst controller. This enables the first controller to compare the dclink voltage demand signal indicative of a desired dc link voltage tothe dc link voltage feedback signal to determine a dc link currentdemand signal and then limit the dc link current demand signal using thecontrol signal from the second controller to determine a limited dc linkcurrent demand signal. The limited dc link current demand signal canthen be used by the first controller to determine the quadrature axiscurrent demand signal for the first active rectifier/inverter.

Alternatively, the second controller may supply a control signal thatvaries in accordance with the prevailing supply network voltageconditions and/or the power demand signal to the first controller. Thisenables a dc link voltage controller of the first controller to comparethe dc link voltage demand signal indicative of a desired dc linkvoltage to the dc link voltage feedback signal to provide an outputsignal that is added to the control signal to determine a dc linkcurrent demand signal. The dc current demand signal can then be used todetermine the quadrature axis current demand signal for the first activerectifier/inverter.

The second controller preferably converts the power demand signalindicative of the level of power to be transferred from the dc link tothe supply network through the second active rectifier/inverter to aquadrature axis current demand signal for the second activerectifier/inverter. The second controller can then control thesemiconductor power switching devices of the second activerectifier/inverter to produce filter/supply network electricalquantities that achieve the desired quadrature axis current for thesecond active rectifier/inverter. The term “filter/supply networkelectrical quantities” is used herein to refer to any and all of theindividual phase voltage magnitude, individual phase current magnitude,phase and frequency in a multi-phase active rectifier/inverter system.The term “multi-phase” refers typically to three-phase but can includeother numbers of phases. The operation of the semiconductor powerswitching devices in the second active rectifier/inverter can also becontrolled using gate drive control signals derived in accordance with aconventional pulse width modulation strategy.

The power demand signal can be converted into the quadrature axiscurrent demand signal by dividing the power demand signal by a signalthat is derived from the voltage at the network terminals of the filter.This signal is preferably the quadrature axis component of the acvoltage that is derived from three phase-voltage measurement on thenetwork side of the filter. Alternatively, the power demand signal maybe converted into the quadrature axis current demand signal by dividingthe power demand signal by a filtered version of the signal that isderived from the voltage at the network terminals of the filter.

The second controller preferably uses a further dc link voltage demandsignal indicative of a desired dc link voltage and compares the furtherdc link voltage demand signal to the dc link voltage feedback signal todetermine an unlimited quadrature axis current demand signal. Theunlimited quadrature axis current demand signal can then be limited todetermine the quadrature axis current demand signal for the secondactive rectifier/inverter. The unlimited quadrature axis current demandsignal can be limited to a value that is determined by a limiting signalthat in turn is preferably derived from the power demand signal.

The unlimited quadrature axis current demand signal can be added to aquadrature axis current feedforward signal that is derived from thefollowing signals: a signal indicative of the generator power, a voltagefeedback signal measured at the network terminals of the filter and again signal that varies in accordance with the prevailing supply networkvoltage conditions.

The signal indicative of the generator power may be supplied to thesecond controller from the first controller. Alternatively, the signalindicative of the generator power minus the output of a PI controller ofa dc link voltage controller of the first controller may be supplied tothe second controller and is used by the second controller only during asupply voltage dip situation.

The second controller can modify the limiting signal that is derivedfrom the power demand signal in accordance with the prevailing supplynetwork voltage conditions. The limiting signal can be modified by thesecond controller in response to deviations in the supply network fromnominal voltage conditions, for example during supply network fault ortransient conditions. This will result in changes of power transfer tothe supply network in order to meet supply network utility requirementssuch as voltage and/or frequency support.

The dc link may include a capacitor. In this case the power convertermay further include a current sensor for measuring the current flowingin the capacitor and providing an output signal. The output signal ofthe current sensor can be subtracted from a signal derived from a signalindicative of the generator power to provide an inferred signal that isadded to the output of a dc link voltage controller of the firstcontroller to determine a dc link current demand signal for the firstactive rectifier/inverter. Alternatively, the output signal of thecurrent sensor can be subtracted from a signal derived from a signalindicative of the generator power to provide a signal that is filteredand added to the output of a dc link voltage controller of the firstcontroller to determine a dc link current demand signal for the firstactive rectifier/inverter.

Alternatively, the power converter may further include a voltage sensorfor measuring the dc link voltage and providing a dc link voltagefeedback signal. Means may also be provided for measuring the rate ofchange of the dc link voltage feedback signal. The integral value of aPI controller of a dc link voltage controller of the first controllercan then be modified by a predetermined factor when the dc link voltagefeedback signal is greater than a first threshold and the rate of changeof the dc link voltage feedback signal is greater than a secondthreshold.

During a supply network voltage dip situation, a quadrature axis currentaxis demand signal for the second active rectifier/inverter may bederived from a slew rate limited version of a signal that is derivedfrom the power limit rating of the second active rectifier/inverter thatis modified as a function of the prevailing supply network voltageconditions.

The second controller preferably compares the voltage demand signalindicative of the level of voltage to be achieved at the networkterminals of the filter to a voltage feedback signal measured at thenetwork terminals of the filter to determine a direct axis currentdemand signal for the second active rectifier/inverter. The secondcontroller can then control the semiconductor power switching devices,ofthe second active rectifier/inverter to produce filter/supply networkelectrical quantities that achieve the desired direct axis current forthe second active rectifier/inverter.

The second controller can modify the direct axis current demand signalin accordance with the prevailing supply network voltage conditions.

The second controller can modify an error signal arising from thedifference between the voltage demand signal indicative of the level ofvoltage to be achieved at the network terminals of the filter and thevoltage feedback signal measured at the network terminals of the filterin accordance with a signal derived from the direct axis current demandsignal. The purpose of modifying the error signal in accordance with asignal derived from the direct axis current demand signal is that acharacteristic can be realised which can contribute to current sharingbetween multiple generators which are connected to a particular supplynetwork.

The power converter preferably further comprises a speed sensor forderiving a speed signal indicative of the speed of the moving part ofthe generator (i.e. the rotor in the case of the rotating generator andthe translator in the case of the linear generator). However, in somecases the speed sensor may be replaced by a speed observer system thatuses internal signals to the first active rectifier/inverter to derive aspeed signal. The speed signal (derived from the speed sensor or thespeed observer system) can then be used to derive the power demandsignal by reference to a look-up table of power demand signal versusspeed. The look-up table may be combined with a PI controller. The speedsignal is preferably modified by a filter function. The speed signal mayalso be modified by a second filter function and multiplied by a gain toprovide a damping term, which is added to the power demand signalderived with reference to the look-up table to give a total power demandsignal. The filter functions may be used independently or together todampen any shaft or drive train resonances if applicable.

The present invention also provides an arrangement comprising aplurality of power converters as described above connected together inparallel to a supply network operating at nominally fixed voltage andnominally fixed frequency by a parallel connection. The voltage demandsignal indicative of the voltage to be achieved at the network terminalsof the filter of each power converter is preferably derived from acomparison of a top-level voltage demand signal and a top-level voltagefeedback signal that is measured at the point where the parallelconnection is connected to the supply network.

Each individual power converter preferably includes a step-uptransformer electrically connected between the associated filter and theparallel connection. The arrangement may also include a step-uptransformer electrically connected between the parallel connection andthe supply network. The top-level voltage feedback signal can bemeasured at either the supply network side or the parallel connectionside of the step-up transformer electrically connected between theparallel connection and the supply network. The advantage of measuringthe top-level voltage feedback signal on the supply network side of thestep-up transformer is that the measurement on the parallel connectionside is subject to regulation across the step-up transformer. Thisregulation effect is therefore eliminated if the measurement is made onthe supply network side.

The power converter is suitable for use in a wind turbine. The presentinvention therefore also provides a wind turbine comprising a generatorhaving a stator and a rotor, a turbine assembly including a turbineblade or turbine blades for rotating the rotor of the generator, and apower converter as described above. The turbine assembly can be integralwith the rotor of the generator. Alternatively, the blade or blades ofthe turbine (three blades might be typical) is mounted to a rotatableshaft and the rotor of the generator is coupled to the rotatable shaft.The rotor of the generator can be coupled directly to the rotatableshaft or indirectly through a gearbox.

A plurality of wind turbines can be connected together to form a windfarm. The present invention therefore further provides a wind farmcomprising a supply network operating at nominally fixed voltage andnominally fixed frequency, and a plurality of wind turbines as describedabove. The respective power converters of the plurality of wind turbinesare connected together in parallel to the supply network by a parallelconnection and the voltage demand signal indicative of the voltage to beachieved at the network terminals of the filter of each power converteris derived from a comparison of a top-level voltage demand signal and atop-level voltage feedback signal that is measured at the point wherethe parallel connection is connected to the supply network.

Each individual wind turbine preferably includes a step-up transformerelectrically connected between the filter of the associated powerconverter and the parallel connection. The wind farm may furthercomprise a step-up transformer electrically connected between theparallel connection and the supply network. The top-level voltagefeedback signal can be measured at either the supply network side or theparallel connection side of the step-up transformer electricallyconnected between the parallel connection and the supply network.

The present invention further provides a method of operating a powerconverter that can be used to interface a generator that providesvariable voltage at variable frequency to a supply network operating atnominally fixed voltage and nominally fixed frequency, the powerconverter comprising:

-   -   a first active rectifier/inverter electrically connected to the        stator of the generator and including a plurality of        semiconductor power switching devices;    -   a second active rectifier/inverter including a plurality of        semiconductor power switching devices;    -   a dc link connected between the first active rectifier/inverter        and the second active rectifier/inverter;    -   a filter connected between the second active rectifier/inverter        and the supply network, the filter including network terminals;    -   a first controller for the first active rectifier/inverter; and    -   a second controller for the second active rectifier/inverter;    -   wherein the method comprises the steps of:    -   the first controller using a dc link voltage demand signal        indicative of a desired dc link voltage to control the        semiconductor power switching devices of the first active        rectifier/inverter to achieve the desired level of dc link        voltage that corresponds to the dc link voltage demand signal;        and    -   the second controller using a power demand signal indicative of        the level of power to be transferred from the dc link to the        supply network through the second active rectifier/inverter, and        a voltage demand signal indicative of the voltage to be achieved        at the network terminals of the filter to control the        semiconductor power switching devices of the second active        rectifier/inverter to achieve the desired levels of power and        voltage that correspond to the power and voltage demand signals.

The method may include further steps as outlined below.

The second controller may use a measure of the supply network voltage todetermine limits on the power that can be exported from the secondactive rectifier/inverter when the supply network voltage deviates fromits nominal condition.

The second controller may also use a measure of the supply networkvoltage to determine the level of current that is to be provided fromthe second active rectifier/inverter to provide voltage support to thesupply network when the supply network voltage deviates from its nominalcondition.

The first controller may use a flux demand signal indicative of adesired level of flux to be achieved in the generator, convert the fluxdemand signal to a direct axis current demand signal for the firstactive rectifier/inverter and control the semiconductor power switchingdevices of the first active rectifier/inverter to produce statorelectrical quantities that achieve the desired direct axis current forthe first active rectifier/inverter. The step of converting the fluxdemand signal to the direct current axis demand signal can be carriedout with reference to one or more characteristics of the generator.

The first controller may compare the dc link voltage demand signalindicative of a desired dc link voltage to a dc link voltage feedbacksignal to determine a quadrature axis current demand signal for thefirst active rectifier/inverter and control the semiconductor powerswitching devices of the first active rectifier/inverter to producestator electrical quantities that achieve the desired quadrature axiscurrent for the first active rectifier/inverter.

The second controller may supply a control signal that varies inaccordance with the prevailing supply network voltage conditions to thefirst controller during a supply network voltage dip situation. Thefirst controller can compare the dc link voltage demand signalindicative of a desired dc link voltage to the dc link voltage feedbacksignal to determine a dc link current demand signal and limit the dclink current demand signal using the control signal from the secondcontroller to determine a limited dc link current demand signal. Thefirst controller can then use the limited dc link current demand signalto determine the quadrature axis current demand signal for the firstactive rectifier/inverter so that no power is drawn from the supplynetwork during the supply network voltage dip situation.

Alternatively, the second controller may supply a control signal thatvaries in accordance with the prevailing supply network voltageconditions and/or the power demand signal to the first controller. A dclink voltage controller of the first controller may then compare the dclink voltage demand signal indicative of a desired dc link voltage tothe dc link voltage feedback signal to provide an output signal that isadded to the control signal to determine a dc link current demandsignal. The dc link current demand signal may then be used to determinethe quadrature axis current demand signal for the first activerectifier/inverter.

The second controller may convert the power demand signal indicative ofthe level of power to be transferred from the dc link to the supplynetwork through the second active rectifier/inverter to a quadratureaxis current demand signal for the second active rectifier/inverter andcontrol the semiconductor power switching devices of the second activerectifier/inverter to produce filter/supply network electricalquantities that achieve the desired quadrature axis current for thesecond active rectifier/inverter.

The above step of converting the power demand signal to the quadratureaxis current demand signal may be carried out by dividing the powerdemand signal by a signal that is derived from the voltage at thenetwork terminals of the filter. Alternatively, the power demand signalmay be converted into the quadrature axis current demand signal bydividing the power demand signal by a filtered version of the signalthat is derived from the voltage at the network terminals of the filter.

The second controller may use a further dc link voltage demand signalindicative of a desired dc link voltage, compare the further dc linkvoltage demand signal to the dc link voltage feedback signal todetermine an unlimited quadrature axis current demand signal and limitthe unlimited quadrature axis current demand signal to a valuedetermined by a limiting signal that is derived from the power demandsignal to determine the quadrature axis current demand signal for thesecond active rectifier/inverter during start-up and the normaloperating condition of the power converter.

The method can further comprise the step of adding the unlimitedquadrature axis current demand signal to a quadrature axis currentfeedforward signal that is derived from the following signals: a signalindicative of the generator power, a voltage feedback signal measured atthe network terminals of the filter and a gain signal that varies inaccordance with the prevailing supply network voltage conditions.

The signal indicative of the generator power can be supplied to thesecond controller from the first controller. Alternatively, the signalindicative of the generator power minus the output of a PI controller ofa dc link voltage controller of the first controller can be supplied tothe second controller and is used by the second controller only during asupply voltage dip situation.

The second controller may modify the limiting signal that is derivedfrom the power demand signal in accordance with the prevailing supplynetwork voltage conditions in a supply network voltage dip situation.The use of the word “dip” in this description in relation to supplynetwork dip situations refers to a situation where the supply networkvoltage is reduced below its nominal value as a result of eithersymmetrical or asymmetrical network fault conditions.

The dc link may include a capacitor and the power converter may furtherinclude a current sensor for measuring the current flowing in thecapacitor and providing an output signal. In this case, the method canfurther comprise the steps of subtracting the output signal of thecurrent sensor from a signal derived from a signal indicative of thegenerator power to provide an inferred signal that is added to theoutput of a dc link voltage controller of the first controller todetermine a dc link current demand signal for the first activerectifier/inverter. Alternatively, the method can further comprise thesteps of subtracting the output signal of the current sensor from asignal derived from a signal indicative of the generator power toprovide a signal that is filtered and added to the output of a dc linkvoltage controller of the first controller to determine a dc linkcurrent demand signal for the first active rectifier/inverter.

Alternatively, the power converter may further include a voltage sensorfor measuring the dc link voltage and providing a dc link voltagefeedback signal and means for measuring the rate of change of the dclink voltage feedback signal. In this case, the method can furthercomprise the steps of modifying the integral value of a PI controller ofa dc link voltage controller of the first controller by a predeterminedfactor when the dc link voltage feedback signal is greater than a firstthreshold and the rate of change of the dc link voltage feedback signalis greater than a second threshold.

In a supply network voltage dip situation, a quadrature axis currentaxis demand signal for the second active rectifier/inverter can bederived from a slew rate limited version of a signal derived from thepower limit rating of the second active rectifier/inverter that ismodified as a function of the prevailing supply network voltageconditions.

The second controller may compare the voltage demand signal indicativeof the level of voltage to be achieved at the network terminals of thefilter to a voltage feedback signal measured at the network terminals ofthe filter to determine a direct axis current demand signal for thesecond active rectifier/inverter and control the semiconductor powerswitching devices of the second active rectifier/inverter to producefilter/supply network electrical quantities that achieve the desireddirect axis current for the second active rectifier/inverter.

The second controller may modify the direct axis current demand signalin accordance with the prevailing supply network voltage conditions.

The second controller may modify an error signal arising from thedifference between the voltage demand signal indicative of the level ofvoltage to be achieved at the network terminals of the filter and thevoltage feedback signal measured at the network terminals of the filterin accordance with a signal derived from the direct axis current demandsignal.

A speed signal indicative of the speed of the moving part of thegenerator may be derived and used to derive the power demand signal. Thespeed signal may be modified by one or more filter functions, which mayalso be used to provide damping of any shaft or drive train resonances.

The power demand signal can be derived from a look-up table ormathematical function where the modified speed signal forms a pointer tothe look-up table or a value for which the mathematical function iscalculated. The power demand signal may also be summed with a signalderived from a filtered version of the speed signal.

The present invention also provides a method of operating a plurality ofpower converters as described above connected together in parallel to asupply network operating at nominally fixed voltage and nominally fixedfrequency by a parallel connection, the method comprising the step ofderiving the voltage demand signal indicative of the voltage to beachieved at the network terminals of the filter of each power converterfrom a comparison of a top-level voltage demand signal and a top-levelvoltage feedback signal that is measured at the point where the parallelconnection is connected to the supply network. The method may alsocomprise the step of measuring the top-level voltage feedback signal ateither the supply network side or the parallel connection side of thestep-up transformer electrically connected between the parallelconnection and the supply network.

The present invention also provides a method of operating a wind turbinecomprising a generator that provides variable voltage at variablefrequency and has a stator and a rotor, a turbine assembly including atleast one blade for rotating the rotor of the generator, and a powerconverter that interfaces the generator to a supply network operating atnominally fixed voltage and nominally fixed frequency, the powerconverter comprising:

-   -   a first active rectifier/inverter electrically connected to the        stator of the generator and including a plurality of        semiconductor power switching devices;    -   a second active rectifier/inverter including a plurality of        semiconductor power switching devices;    -   a dc link connected between the first active rectifier/inverter        and the second active rectifier/inverter;    -   a filter connected between the second active rectifier/inverter        and the supply network, the filter including network terminals;    -   a first controller for the first active rectifier/inverter; and    -   a second controller for the second active rectifier/inverter;    -   wherein in response to a change in wind speed method comprises        the steps of:    -   controlling the second active rectifier/inverter to change the        level of power exported out of the dc link such that the dc link        voltage changes from a desired level; and    -   controlling the first active rectifier/inverter to import        sufficient current into the dc link through the generator bridge        from the generator to restore the dc link voltage to the desired        level.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing showing how a power converter according tothe present invention is used to interface between a wind turbinedriving a variable speed generator and a fixed frequency power network;

FIG. 2 is a schematic drawing showing more detail of the dc link controlfor the generator bridge (active rectifier) of FIG. 1;

FIG. 3 is a schematic drawing showing more detail of the current controlfor the generator bridge (active rectifier) of FIG. 1;

FIG. 4 is a schematic drawing showing more detail of the power controlfor the network bridge (inverter) of FIG. 1;

FIG. 5 is a schematic drawing showing more detail of the current controlof the network bridge (inverter) of FIG. 1;

FIG. 6 is a schematic drawing showing how a number of power convertersaccording to the present invention can be connected together in parallelto the supply network to form a wind farm;

FIG. 7 is a schematic drawing showing an overall wind farm voltagecontrol;

FIG. 8 is a schematic drawing showing how a first alternative powerconverter according to the present invention is used to interfacebetween a wind turbine driving a variable speed generator and a fixedfrequency power network;

FIG. 9 is a schematic drawing showing more detail of the dc link voltageand subordinate current controls for the generator bridge (activerectifier) of FIG. 8;

FIG. 10 is a schematic drawing showing more detail of the power control,network voltage control and subordinate current controls for the networkbridge (inverter) of FIG. 8;

FIG. 11 is a schematic drawing showing how a second an alternative powerconverter according to the present invention is used to interfacebetween a wind turbine driving a variable speed generator and a fixedfrequency power network;

FIG. 12 is a schematic drawing showing more detail of a first option fordc link control for the generator bridge (active rectifier) of FIG. 11;

FIG. 13 is a schematic drawing showing more detail of the power controlfor the network bridge (inverter) of FIG. 11; and

FIG. 14 is a schematic drawing showing more detail of a second optionfor dc link control for the generator bridge (active rectifier) of FIG.11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Power Converter Topology

The basic topology of the power converter will be outlined withreference to FIG. 1.

The power converter is used to interface between a wind turbine 2driving a variable speed ac induction generator 4 and a nominally fixedfrequency power network (labelled NETWORK). The wind turbine typicallyincludes three turbine blades (one turbine blade or two turbine bladesor more than three turbine blades are also possible) mounted on arotating shaft and whose pitch can be controlled by means of a pitchactuator in order to optimise and/or limit the capture of wind energyinto the generator 4. A gearbox 8 is used to connect the rotating shaftto the rotor of the variable speed generator 4. In some cases, therotating shaft can be connected directly to the rotor of the variablespeed generator. This means that the speed of rotation of the rotorvaries as a function of the wind speed and that the frequency of thevoltage developed at the stator of the generator 4 (the “statorfrequency”) may therefore vary over wide ranges. A number of windturbines as represented by the entirety of FIG. 1 can be connectedtogether to define a wind farm.

The terminals of the generator 4 are connected to the ac terminals of athree-phase generator bridge 10 which in normal operation operates as anactive rectifier to supply power to a dc link 12. The generator bridge10 has a conventional three-phase two-level topology with a series ofsemiconductor power switching devices fully controlled and regulatedusing a pulse width modulation strategy. However, in practice thegenerator bridge 10 can have any suitable topology such a three-levelneutral point clamped topology or a multi-level topology (Foch-Maynardarrangement, for example). The derivation of the gate drive commandsignals that are used to control the semiconductor power switchingdevices is described in more detail below.

The dc output voltage of the generator bridge 10 is fed to the dcterminals of a network bridge 14 which in normal operation operates asan inverter. The principle control for the dc output voltage is achievedby controlling the generator bridge 10. The network bridge 14 has asimilar three-phase two-level topology to the generator bridge 10 with aseries of semiconductor power switching devices fully controlled andregulated using a pulse width modulation strategy. However, in practicethe network bridge 14 can have any suitable topology, as discussed abovefor the generator bridge 10. The network bridge 14 is controlled to meettwo principle objectives, namely active power and network voltage. Adetailed description of how this control is achieved is provided below.The derivation of the gate drive command signals that are used tocontrol the semiconductor power switching devices is also described inmore detail below.

As described herein, active rectification (as the prime mode ofoperation of the generator bridge 10) is the conversion of energy fromthe ac terminals of the three-phase generator bridge to the dc link andinversion (as the prime mode of operation of the network bridge 14) isthe conversion of energy from the dc link of the three-phase networkbridge to its ac terminals. However, it will be readily appreciated thatthere may be times when it might be necessary or desirable to operatethe generator bridge 10 as an inverter and operate the network bridge 14as an active rectifier. For example, during start-up the network bridge14 will operate as an active rectifier to supply power from the supplynetwork to the dc link 12. In situations where a network voltage dipoccurs, the generator bridge 10 may operate in either an activerectifier mode or in an inverter mode as required in order to controlthe voltage of the dc link 12. The action of controllers for thegenerator bridge 10 and the network bridge 14 (that is the generatorbridge controller 18 and the network bridge controller 46 described inmore detail below) is coordinated in the event of a network voltage dipsuch that power is not drawn from the supply network but, subject to theparameterisation and the level of the voltage dip, the power converteris still capable of supplying power to the supply network.

It can also be advantageous for maintenance purposes and when the windturbine is operating at very low speeds to operate the generator 4 in amotoring mode. In this case, power can be supplied from the supplynetwork to the generator 4 through the network bridge 14 operating as anactive rectifier and the generator bridge 10 operating as an inverter.

The ac output voltage of the network bridge 14 is filtered by inductors16 (and possible other filters) and supplied to the nominally fixedfrequency power network via a step-up transformer 6. Protectiveswitchgear (not shown) can be included to provide a reliable connectionto the power network and to isolate the generator system from the powernetwork for various operational and non-operational requirements.

Wind Farm Topology

As mentioned briefly above, a number of wind turbines as represented bythe entirety of FIG. 1 can be connected together to define a wind farm.This is shown schematically in FIG. 6 where a number of power converters1 a to 1 d are connected to the nominally fixed frequency supply network(labelled NETWORK) by a parallel connection 72. Each power converter 1 ato 1 d includes a filter 16 a to 16 d and a step-up transformer 6 a to 6d. An additional wind farm step-up transformer 74 is also providedbetween the parallel connection 72 and the supply network. FIG. 6 showshow the wind farm voltage feedback signal that is described in moredetail below with reference to FIG. 7 can be measured at the parallelconnection side (labelled WINDFARM VOLTAGE FEEDBACK A) or the supplynetwork side (labelled WINDFARM VOLTAGE FEEDBACK B) of the wind farmstep-up transformer 74. The advantage of measuring the top-level voltagefeedback signal on the supply network side of the wind farm step-uptransformer 74 is that the measurement on the parallel connection sideis subject to regulation across the step-up transformer. This regulationeffect is therefore eliminated if the measurement is made on the supplynetwork side. Alternatively, the measurement of the wind farm voltagefeedback signal at the supply network side can be calculated using themeasurement of the wind farm voltage feedback signal at the parallelconnection side, the characteristics of the wind farm step-uptransformer 74 and the amplitude and angle of the current through thewind farm step-up transformer.

Generator Bridge Control

The control of the generator bridge 10 will now be explained withreference to FIGS. 1 to 3.

A generator bridge controller 18 receives a dc link voltage demandsignal VDC₁₃ GEN* and a voltage feedback signal VDC_FB indicative of thedc link voltage. VDC_FB is subtracted from VDC₁₃ GEN* and the differenceis supplied to a PI controller 20 with variable integral gain Ki andproportional gain Kp inputs to provide a dc link current demand signalIDC_GEN* that is the effective current required to flow in the dc link12 to satisfy the prevailing operational conditions. This dc linkcurrent demand signal IDC_GEN* is then limited during grid faultconditions by a signal IDC_LIM supplied from the network bridgecontroller 46 (see below) to form a signal IDC_GEN*_LIM. To convert thelimited dc link current demand signal IDC_GEN*_LIM into a quadratureaxis current demand signal IQ_GEN* relating to the generator phasecurrent then the limited dc link current demand signal IDC_GEN*_LIM isfirst multiplied by the voltage feedback signal VDC_FB to provide apower signal POWER_GEN. The power signal POWER_GEN is then converted inthe IQ_GEN* calculator function block 92 to the quadrature axis currentdemand signal IQ_GEN* by applying the following formula:${IQ\_ GEN}^{*} = \frac{\left( \frac{POWER\_ GEN}{\sqrt{3}} \right) - \left( {{VD\_ FF} \times {ID\_ GEN}^{*}} \right)}{VQ\_ FF}$where VD_FF is the feedforward component of the direct voltage within acurrent controller 26 of FIG. 2, ID_GEN* is the direct axis demandcurrent supplied from a saturation characteristic function block 32 andVQ_FF is the feedforward component of the quadrature axis voltage withinthe current controller 26.

The quadrature axis current demand signal IQ_GEN* is constrained by alimit function to remain within the non-breakout region of the generatorcharacteristic and the voltage and current ratings of the generator andnetwork bridges. This limit is determined by an off-line calculation tocreate a look-up table embedded in the functional block 22 based onmachine equivalent circuit parameters, drive rating parameters andrequired operational speed range. The resulting look-up table is usedduring the operation of the power converter by accessing it with a rotorspeed feedback signal N (or an observed rotor speed signal) and takingthe resulting signal as the limit value for the IQ_GEN* limit functionblock 24. The resulting limited quadrature axis current demand signalIQ_GEN*_LIM is then supplied to a current controller 26 (described inmore detail below). The limited quadrature axis current demand signalIQ_GEN*_LIM is also used to determine the slip frequency WS that is tobe applied to the generator 4 to achieve the necessary power flow fromthe generator to the dc link 12. The slip frequency WS is determinedusing the following function:${WS} = \frac{{IQ\_ GEN}^{*}{\_ LIM} \times {RR} \times {LM}}{\Phi^{*} \times {LR}}$

Where RR is the rotor resistance, LM is the magnetising inductance, Φ*is the generator flux demand signal and LR is the rotor leakageinductance.

Integrating the slip frequency WS provides an output θS, which is theslip angle. Integrating the output from a speed observer 28 provides θR,which is the observed rotor angle. (The observer function 28 can bereplaced by direct measurement of the rotor position by the use of anincremental encoder or similar device.) A rotor flux angle θ0 can thenbe determined by summing the slip angle θS and the rotor angle θR. Therotor flux angle θ0 is the angle at which the combination of the directaxis voltage VD and the quadrature axis voltage VQ are to be applied atthe stator terminals of the generator 4 by means of a pulse widthmodulation generator 30. This is described in more detail below. Itshould be noted that for synchronous generators the step of defining andintegrating slip frequency is not required.

The generator flux demand signal Φ* (which can be constant or variabledepending on the required system characteristics) is applied to afunction block 32 containing the saturation characteristic of thegenerator magnetising inductance. The saturation characteristic isdetermined either by direct measurement when the generator iscommissioned or by extracting data from the factory test results for thegenerator. The output of the saturation characteristic function block 32is a magnetising current signal and becomes the direct axis currentdemand signal ID_GEN* applied to the current controller 26. Forsynchronous generators, the direct axis current demand signal isdetermined by the generator terminal voltage requirements for each speedand load condition. By adjusting the direct axis current demand signalto a synchronous generator the excitation can be modified by the actionof the generator bridge 10 to optimise the terminal voltage and theoverall generator efficiency for each operational condition.

The current controller 26 for the generator bridge 10 includes tworegulators, one operating in the direct current axis and one operatingin the quadrature current axis. Overall, the current controller 26operates in a synchronous reference frame aligned with the rotor fluxangle. FIG. 3 shows the overall direct and quadrature axis currentregulators of the generator bridge 10.

In addition to the limited quadrature axis current demand signalIQ_GEN*_LIM and the direct axis current demand signal ID_GEN*, thecurrent controller 26 is also supplied with a quadrature axis currentfeedback signal IQ_GEN and a direct axis current feedback signal ID_GENthat are derived from the measurement of the generator phase currentsIU, IV and IW. The conversion from three-phase components in astationary reference frame to direct/quadrature components in asynchronous reference frame is achieved using a combined Clarke/Parktransform block 34. The transform uses the rotor flux angle θ0 for theconversion. It can be seen from FIG. 3 that the current controller 26also receives the following additional signals: the generator fluxdemand signal Φ* (which can be constant or variable depending on therequired system characteristics) and the generator stator frequency W0.The stator frequency W0 is calculated from the sum of the slip frequencyand the rotor frequency. Rotor frequency is derived from the observedrotor speed and the pole number of the generator.

The current controller 26 operates by comparing the direct axis currentdemand signal ID_GEN* with the direct axis current feedback signalID_GEN and the limited quadrature axis current demand signal IQ_GEN*_LIMwith the quadrature axis current feedback signal IQ_GEN and applying theresulting errors to independent PI controllers. The outputs from the PIcontrollers are then summed with cross-coupling signals derived from theproduct of current demands and machine parameters to produce a totaloutput voltage for the direct and quadrature axes, VD_GEN* and VQ_GEN*respectively. The cross-coupling terms are shown in FIG. 3 and emulatethe standard voltage equations for the generator 4 in the steady state.With reference to the cross-coupling terms, δLS is the generator statorleakage inductance and RS is the generator stator resistance.

The final voltage outputs from the current controller 26, VD_GEN* andVQ_GEN* are converted from Cartesian to polar co-ordinates using aco-ordinate converter 38. The total voltage magnitude V_GEN* iscalculated according to the equation:V_GEN*=√{square root over ((VD_GEN*² +VQ_GEN*²))}and supplied to the gate drive command signal controller 36. The anglebetween the total voltage magnitude V_GEN* and the quadrature axisvoltage VQ_GEN* is θ_GEN and is calculated from the arctangent ofVD_GEN*/VQ_GEN* as follows:${\theta\_ GEN} = {{arc}\quad{\tan\left( \frac{{VD\_ GEN}^{*}}{{VQ\_ GEN}^{*}} \right)}}$

The angle θ_GEN between the total voltage magnitude V_GEN* and thequadrature axis voltage VQ_GEN* is added to the rotor flux angle θ0 todetermine the angle at which the total voltage is to be impressed on thestator terminals of the generator 4.

The individual upper (U) and lower (L) gate drive command signals forthe three-phases U, V and W resulting in individual signals UU, UL, VU,VL, WU and WL of the generator bridge 10 are calculated in the pulsewidth modulation (PWM) generator 30 using the total voltage magnitudeV_GEN*, the sum of the angles θ_GEN and θ0 and the pulse widthmodulation frequency. The dc link voltage feedback signal VDC_FB is alsofactored into these PWM calculations. The dc link voltage feedbacksignal VDC_FB can be derived independently when independent controllersare used for the generator bridge 10 and the network bridge 14,respectively. This is particularly necessary when the generator bridge10 and the network bridge 14 are physically remote from each other and asignificant inductance exists between the dc link capacitance of eachbridge. In situations where an independently-derived dc link voltagefeedback signal is provided for each bridge then it will be readilyappreciated that the following substitution should be made:For the generator bridge 10: VDC_FB=VDC_FB_GENFor the network bridge 14: VDC_FB=VDC_FB_NET

The current controller 26 also produces a power feedforward signalindicative of the generator power POWER_FF, which is calculated asfollows:POWER_(—) FF=√{square root over (3)}( VQ_GEN*×IQ_GEN+VD_GEN*×ID_GEN)

This is used as a feedback signal to the network bridge controller 46.

Network Bridge Control

The control of the network bridge 14 will now be explained withreference to FIGS. 1 and 4 to 7. The control is based on a voltagecontrol scheme and is different from the power factor angle controlscheme and reactive power control scheme used in the conventional powerconverters described above.

The voltage control scheme includes two levels of control. Withreference to FIG. 7, the first is defined at the wind farm level and isresponsive to a wind farm voltage demand signal that is typically set bythe utility company who controls the wind farm. This wind farm voltagedemand signal is compared to a wind farm voltage feedback signal and theerror between the two signals is applied to a proportional plus integralcontroller 40 to define a turbine voltage demand signal VTURB* that istransmitted to all of the wind turbines T1 to TN in the wind farm. Asecond level of control is then applied to each of the individual windturbines to regulate its own output voltage in response to the turbinevoltage demand signal VTURB*.

With reference to FIG. 4, in each of the wind turbines of the wind farm,the turbine voltage demand signal VTURB* is compared in a summing node42 to a quadrature axis voltage subordinate feedback signal VQ_NET (seebelow) that is derived from three phase-voltage measurement on thenetwork side of the inductors 16. The difference between the two signalsis fed to a PI controller 44 to form a reactive current demand signalID_NET* that is supplied via a limitation block 66 to a currentcontroller 58 described in more detail below.

The reactive current demand signal ID_NET* is also fed back through aproportional gain controller 48 to the summing node 42 to further modifythe voltage difference signal. This serves to provide a droopcharacteristic, such that when multiple wind turbines are connectedtogether in parallel to a wind farm transformer through differentconnecting impedances, the reactive current sharing between each windturbine is more balanced. The droop gain can be adjusted depending onsite network configurations to give adequate current balance between thewind turbines and to respect rating limitations. Limits are applied tothe direct and quadrature axis current demand signals ID_NET* andIQ_NET*, respectively, as described below for network voltage dipsituations.

The voltage control scheme is integrated with the network bridgecontroller 46 as follows. The network bridge controller 46 has fiveprinciple input signals and seven principle feedback signals and usesthese to derive gate drive command signals to control the operation ofthe semiconductor power switching devices in the network bridge 14.

The input signals include a dc link voltage demand signal for thenetwork bridge VDC_NET*, a power export demand signal P*, the turbinevoltage demand signal VTURB*, a parameter DRIVE RATING defining thedrive current rating and the power feedforward signal POWER_FF suppliedfrom the generator bridge controller 18 and which is indicative of thegenerator power. The feedback signals include three phase voltagemeasurements VRY, VYB and VBR (that is the voltage measurements takenacross the so-called red (R), yellow (Y) and blue (B) output lines thatsupply power from the network bridge 14 to the network), three phasecurrent measurements IR, IY and IB, and the voltage feedback signalVDC_FB indicative of the dc link voltage. The feedback signals are usedto derive the following voltage and current subordinate feedback signalsfor the network bridge 14 in the direct and quadrature axes: VD_NET,VQ_NET, ID_NET and IQ_NET. In addition, a control signal IDC_LIM ispassed from the network bridge controller 46 to the generator bridgecontroller 18 to permit fast power reduction and coordinated controlbetween the controllers during grid fault conditions. During such gridfault conditions, the dc link voltage control is distributed between thenetwork and generator bridges such that no active power is drawn fromthe supply network and the required supply network voltage support andpower export requirements are achieved.

Function block 68 incorporates a phase locked loop (PLL) system toderive the signal θMAINS, which is a measure of the network voltageangle.

The dc link voltage demand signal VDC_NET* is only needed to meetstart-up requirements, to maintain connection with the network duringzero wind conditions and permit fast coordinated control of the dc linkvoltage between the generator bridge controller 18 and the networkbridge controller 46 during grid fault conditions. In operation, thevoltage feedback signal VDC_FB is subtracted from the dc link voltagedemand signal VDC_NET* and the result is applied to a PI controller 50to determine the signal VDC_PI_IQ_NET*. A signal IQ_FF indicative of thequadrature axis network current required to export the instantaneousgenerator power is calculated in function block 71 from the powerfeedforward signal POWER_FF, a signal representing the network voltageVQ_NET and a gain signal PFF_GAIN that is an output of the limitationblock 66. This is then added to the signal VDC_PI_IQ_NET* to create anunlimited signal IQ_NET*. The resulting signal is constrained by a limitfunction (limit function block 52) driven by the lesser of P*/VQ_NET orthe limit derived from the network voltage dip requirements.

With reference to FIG. 1, the rotor speed feedback signal N is derivedfrom a speed sensor 54 (or alternatively from an observed rotor speedsignal) and then filtered to provide a first filtered speed signal N′and a second filter speed signal N′2. The second filtered speed signalN′2 provides damping for any shaft resonance via a damping gain KD. Thefirst filtered speed signal N′ provides a pointer to a pre-calculatedlook-up table 56 of power demand versus filtered speed. The look-uptable may be combined with a PI controller. The resulting power exportdemand signal P*, which is the sum of the damping and look-up tablepower demand signals, is applied to the network bridge controller 46 asshown in FIG. 1. More particularly, the power export demand signal P* isdivided by the quadrature axis voltage subordinate feedback signalVQ_NET to become the limiting signal for the quadrature axis currentdemand signal IQ_NET* under normal operating conditions. Alternatively,the power export demand signal P* may be converted into the quadratureaxis current demand signal IQ_NET* by dividing the power export demandsignal P* by a filtered version of the quadrature axis voltagesubordinate feedback signal VQ_NET that is derived from the voltage atthe network terminals of the inductors 16.

The limited quadrature current requirement signal IQ_NET*_LIM (that isthe output of the limit function block 52) is the input to a currentcontroller 58. The current controller 58 for the network bridge 14includes two regulators, one operating in the direct axis and oneoperating in the quadrature axis. Overall, the current controller 58operates in a synchronous reference frame aligned with the quadratureaxis network voltage VQ_NET. FIG. 5 shows the overall direct andquadrature current axis current regulators of the network bridge 14.

In addition to the limited quadrature axis current demand signalIQ_NET*_LIM and a limited direct axis current demand signal ID_NET*_LIM(that is the output of the limitation block 66), the current controller58 is also supplied with a quadrature axis current feedback signalIQ_NET and a direct axis current feedback signal ID_NET that are derivedfrom the measurement of the network bridge phase currents IR, IY and IB.The conversion from three-phase components in a stationary referenceframe to direct/quadrature components in a synchronous reference frameis achieved using a combined Clarke/Park transform block 70. Thetransform uses the network voltage angle θMAINS for the conversion.

The current controller 58 operates by comparing the limited direct axiscurrent demand signal ID_NET*_LIM with the direct axis current feedbacksignal ID_NET and the limited quadrature axis current demand signalIQ_NET*_LIM with the quadrature axis current feedback signal IQ_NET andapplying the resulting errors to independent PI controllers. The outputsfrom the PI controllers are then summed with cross-coupling signalsderived from the product of current demands and network side circuitimpedance values to produce a total output voltage for the direct andquadrature axes, VD_NET* and VQ_NET* respectively. The cross-couplingterms are shown in FIG. 5 and emulate the standard voltage equations forthe overall network circuit in the steady state. With reference to thecross-coupling terms, LN is the network filter inductance and WN is thefrequency of the network voltage waveform.

The final voltage outputs from the current controller 58, VD_NET* andVQ_NET* are converted from Cartesian to polar co-ordinates using aco-ordinate converter 64. The total voltage magnitude V_NET* iscalculated according to the equation:V_NET*=√{square root over ((VD_NET*² +VQ_NET*²))}and supplied to the gate drive command signal controller 62. The anglebetween the total voltage magnitude V_NET* and the quadrature axisvoltage VQ_NET* is θ_NET and is calculated from the arctangent ofVD_NET*/VQ_NET* as follows:${\theta\_ NET} = {{arc}\quad{\tan\left( \frac{{VD\_ NET}^{*}}{{VQ\_ NET}^{*}} \right)}}$

The angle θ_NET between the total voltage magnitude V_NET* and thequadrature axis voltage VQ_NET* is added to the network voltage angleθMAINS to determine the angle at which the total voltage is to beimpressed by the network bridge 14 on the total network side circuit.

The individual upper (U) and lower (L) gate drive command signals forthe three-phases R, Y and B resulting in individual signals RU, RL, YU,YL, BU and BL of the network bridge 14 are calculated in the pulse widthmodulation generator 60 using the total voltage magnitude V_NET*, thesum of the angles θ_NET and θMAINS and the pulse width modulationfrequency. The dc link voltage feedback signal VDC_FB is also factoredinto these PWM calculations. The dc link voltage feedback signal VDC_FBcan be derived independently when independent controllers are used forthe network bridge 14 and the generator bridge 10, respectively. This isparticularly necessary when the generator bridge 10 and the networkbridge 14 are physically remote from each other and a significantinductance exists between the dc link capacitance of each bridge. Insituations where an independently-derived dc link voltage feedbacksignal is provided for each bridge then it will be readily appreciatedthat the following substitution should be made:For the network bridge 14: VDC_FB=VDC_FB_NETFor the generator bridge 10: VDC_FB=VDC_FB_GEN

In situations where a network voltage dip occurs, the limitation block66 calculates the respective allocation of available current from thenetwork bridge 14, based on its thermal limits, to the quadrature anddirect axes and also calculates the maximum generator dc link currentIDC_LIM. The signal IDC_LIM supplied from the network bridge controller46 to the generator bridge controller 18 is used to rapidly set thelevel of current that can be provided by the generator bridge 10 to theintermediate dc link 12.

Different requirements exist within the various network codes givingpriority to active or reactive current output and the percentages ofreactive current required as a function of dip magnitude. In otherwords, the behaviour of the power converter depends on how it isparameterised for operation in different countries or regions.

Overall, the operation of the power converter is fundamentally differentfrom the operation of the conventional power converters described abovebecause it maintains control of the dc link 12 during network voltagedips by directly controlling the power flow from the generator 4. Bymaintaining control of the dc link voltage during a network voltage dipit is possible to maintain the required reactive current output from thenetwork bridge 14 to meet the voltage support requirements of the powernetwork.

During a network voltage dip, the network bridge dc link voltagecontroller (a combination of the PI controller 50 and the precedingsumming node) becomes the master of the power converter system andallocates both power limit and dc link current limit signals to both thenetwork bridge 14 and the generator bridge 10, respectively, on thebasis of the magnitude of the network voltage dip.

The export of power from the dc link 12 is determined by powerreferencing applied to the network bridge 14. As more power is exportedfrom the dc link 12 (to discharge it) then the generator bridge 10 willreact to this to take more power from the generator 4 to refill the dclink. This is in direct contrast to conventional four-quadrant powerconverters where power is loaded into the dc link to increase the dclink voltage as a result of the torque demand applied to the generatorbridge. Export of power to the network is then determined by the actionof the network bridge controller when the dc link voltage exceeds thenetwork bridge voltage demand.

Operation of the Power Converter

One possible operational implementation of the above power convertertopology is as follows. At start-up, the dc link voltage demand signalVDC_NET* is set to 1050 volts. The semiconductor power switching devicesin the network bridge 14 are enabled and, under control of the networkbridge controller 46, bring the dc link voltage up to 1050 volts. Thiswill almost always require an importation of power from the supplynetwork to the dc link 12 so the quadrature axis current demand outputsignal IQ_NET* will result in power flow into the dc link in thisstart-up condition.

At the same time, the dc link voltage demand signal VDC₁₃ GEN* appliedto the generator bridge power controller 18 is set to 1100 volts.

Assuming that the wind is blowing and the wind turbine 2 is rotating,when the generator bridge 10 is enabled it will control the direct axiscurrent ID_GEN to achieve the necessary magnetic flux in the generator 4for the prevailing speed conditions, and the quadrature axis currentIQ_GEN will be adjusted under the control of the generator bridge 10 toachieve the objective of a dc link voltage of 1100 volts.

As the dc link voltage increases to achieve the objective of 1100 voltsit will exceed the dc link voltage demand signal VDC_NET* for thenetwork bridge 14. As a result, the error signal derived by the networkbridge controller 46 when subtracting the dc link voltage demand signalVDC_NET* from the voltage feedback signal VDC_FB will act such thatpower is transferred from the dc link 12 into the supply network, themagnitude of this power transfer being limited (the limit function block52) by a signal derived from the power export demand signal P*. Thespeed sensor signal N is filtered to provide a first filtered speedsignal N′ and a second filtered speed N′2. The damping gain KD appliedto the second filtered speed signal N′2 provides damping of shaftresonance in the turbine drive train. The first filtered speed signal N′is used as the pointer to a pre-calculated P* versus N′ look-up table56. The power export demand signal P* derived from the look-up table 56is applied to the power controller 46 for the network bridge 14. Theapplied power export demand signal P* is divided by the prevailingquadrature axis network voltage VQ_NET to obtain a limit signal to applyto the quadrature axis current demand output signal IQ_NET* derived fromthe dc link voltage demand signal VDC_NET* for the network bridge 14.

In the event of a network voltage dip, the allocation of rated outputpower (VA) to the active and reactive axes of the network bridgecontroller 46 will be determined in line with the requirements of thespecific network code for which the wind turbine is parameterised. TheApparent Power Limit is calculated from the prevailing voltage V_NET asmeasured by the network voltage feedback circuits and the networkinverter overload current rating I_OVERLOAD. More particularly:Apparent Power Limit=√{square root over (3)}(V_NET×I_OVERLOAD)

In FIG. 4, the input DRIVE RATING is equivalent to I_OVERLOAD in theabove equation.

The power converter operates in a dynamic manner to accommodate changesin wind speed. For example, for an increasing wind speed the speed ofrotation of the wind turbine 2 will also increase hence providing anincreasing power export demand signal P* to the network bridgecontroller 46. The network bridge controller 46 causes the networkbridge 14 to export more power from the dc link 12 to the supplynetwork. Increasing the amount of power that is exported to the supplynetwork leads to a drop in the dc link voltage. The generator bridgecontroller 18 responds to this drop in the dc link voltage by the actionof the dc link voltage controller 76 (comprising the PI controller 20and the preceding summing node) to cause the generator bridge 10 to drawmore power from the generator 4 to provide more current into the dc link12 until a new steady state is achieved (i.e. where the amount of powerthat is supplied to the supply network from the network bridge 14 isequal to the amount of power that is supplied to the generator bridge 10from the generator 4). In this steady state, the dc link voltage hasmatched the dc link voltage demand signal VDC₁₃ GEN*. For reduction inwind speed, the opposite control actions take place.

During the same conditions of an increasing wind speed, the powerconverter described in U.S. Pat. No. 5,083,039 modifies a torque demandsignal that is supplied to the generator bridge controller to cause anincreasing generator torque and hence increasing power flow from thegenerator through the generator bridge to the dc link. This causes anincrease in the dc link voltage. The network bridge controller thenresponds to the increase in the dc link voltage by the action of its dclink voltage controller to increase the amount of power that is exportedto the supply network and hence reduce the dc link voltage back to itsreference value.

U.S. Pat. No. 5,083,039 therefore describes a situation where more powerflow is “pushed” through the power converter from the generator into thedc link in response to an increase in wind speed, and the secondaryresponse is to export power from the dc link to the supply networkthrough the network bridge. However, the power converter of the presentinvention operates in an opposite manner such that, in response to anincrease in wind speed, the more power is “pulled” out of the dc link 12by the network bridge 14, and the secondary response is to import powerto the dc link from the generator 4 through the generator bridge 10 toachieve more current in the dc link.

Alternative Power Converter Topologies

The basic topologies of two different alternative power converterarrangements will be outlined with reference to FIGS. 8 to 14. Thealternative power converters are very similar to the power converter ofFIG. 1 and like parts have been given the same reference numerals. Thepurpose of the topologies of the alternative power converters is toeliminate one or up to three features of the power converter of FIG. 1,namely (i) the dc link voltage demand signal VDC_NET* for the networkbridge 14 and its associated voltage feedback signal VDC_FB and PIcontroller 50, (ii) the signal IDC_LIM that is supplied from the networkbridge controller 46 and used to limit the dc link current demand signalIDC_GEN* during grid fault conditions, and (iii) the power feedforwardsignal POWER_FF that is produced by the current controller 26.

The first alternative power topology eliminates only the features of thedc link voltage demand signal VDC_NET* for the network bridge 14, itsassociated voltage feedback signal VDC_FB and PI controller 50. It alsomodifies the action of the signal IDC_LIM by replacing this with a newfeedforward signal IDC_FF. A modified power feedforward signal POWER_FF′remains active but is only used by the network bridge controller 46during a supply network voltage dip situation. In this case only, themodified power feedforward signal POWER_FF′ is calculated from thestandard power feedforward signal POWER_FF described elsewhere minus theoutput of the PI controller 20 of the dc link voltage controller 76.This is shown in FIG. 9. The modified power feedforward signal POWER_FF′is used in the network bridge controller 46 together with a signal(labelled IQ_CAPACITY) relating to the IQ capacity of the network bridge14 during a supply network voltage dip situation, the power limitPOWER_LIMIT and a signal relating to the amplitude of the prevailingnetwork voltage VQ_NET to calculate a limited quadrature axis currentdemand signal IQ_NET*_LIM that is used during a supply network voltagedip situation. The normal source for the signal IQ_NET*_LIM is ignoredin this situation.

In topologies where the dc link voltage demand signal for the networkbridge VDC_NET* is eliminated then the network bridge 14 can beenergised using the network voltage. The dc link voltage is determinedby the rectified value of the network voltage, which is nominally √2×VLL(i.e. the line to line voltage at the ac terminals of the network bridge14). This establishes a dc power supply from which auxiliary circuits,such as the microprocessor(s) for the generator bridge controller 18 andthe network bridge controller 46, and the gate drive power for thegenerator bridge 10 and network bridge 14, can be derived. The dc linkvoltage is then available to provide fluxing of the generator 4 andbring it under control.

Assuming that the wind is blowing and the wind turbine 2 is rotatingthen the generator 4 can start to provide power to the dc link 12 andachieve a dc link voltage that is equal to the dc link voltage demandsignal VDC₁₃ GEN*.

The basic topology of the first alternative power converter arrangementwill now be described with reference to FIGS. 8 to 10. In thisarrangement, the dc link voltage controller 76 of the generator bridgecontroller 18 remains active under all operating conditions. In steadystate conditions, the action of the integral term within the PIcontroller 20 of the dc link voltage controller 76 is minimised by theinclusion of the feedforward signal IDC_FF from the network bridgecontroller 46. In supply network voltage dip situations, the feedforwardsignal IDC_FF provides information about the amount of dc current to beprovided by the generator bridge 14 in response to changes in the supplynetwork voltage. The signal IDC_FF is calculated in the function block90. By including these features, the variation in the dc link voltageduring supply network voltage dip situations is minimised. Also theaction required by the integral term within the PI controller 20 of thedc link voltage controller 76 is minimised and therefore requires muchsmaller deviations in the actual dc link voltage to increase or decreasethe integral value to the correct value to achieve steady stateoperation.

The basic topology of the second alternative power converter arrangementwill now be described with reference to FIGS. 11 to 14. In thisarrangement, the dc link voltage of the network bridge controller 46 iseliminated in the same manner as described above.

A first option for dc link control for the generator bridge 10 of thesecond alternative power controller arrangement will now be describedwith reference to FIGS. 11 and 12. The purpose of the signal IDC_LIM inthe power converter of FIG. 1 is to pass critical information about theprevailing network voltage conditions and power throughput levels to thegenerator bridge controller 18. This is particularly important during asupply network voltage dip situation when the power throughputcapability is severely limited. In the alternative power converter thesignal IDC_LIM is replaced by an inferred signal IDC_NET′ that is usedonly by the generator bridge controller 18.

The inferred signal IDC_NET′ is calculated from information that isavailable to the generator bridge controller 18 by the followingequation:${IDC\_ NET}^{\prime} = {\left( \frac{POWER\_ FF}{VDC\_ FB} \right) - {I\_ CAP}}$

In the alternative power converter, the power feedforward signalPOWER_FF is derived from the current controller 26 of the generatorbridge controller 18 as illustrated in FIG. 3 using the equation:POWER_(—) FF=√{square root over (3)}( VQ_GEN*×IQ_GEN+VD_GEN*×ID_GEN)

However, the power feedforward signal POWER_FF is not supplied to thenetwork bridge controller 46 but is used only by the generator bridgecontroller 18 in the derivation of the inferred signal IDC_NET′.(POWER_FF has the same derivation here as the POWER_FF of the firstarrangement shown in FIG. 2. The signal name is kept the same forconsistency in this specification, however the POWER_FF in this instanceis not a literal power feedforward signal.) The inferred signal IDC_NET′is used to indicate the effective dc current that the network bridge 14is exporting to the supply network but is calculated from the conditionsof the generator bridge 10.

Dividing the generator power signal POWER_FF by the voltage feedbacksignal VDC_FB gives the effective dc current being supplied to the dclink 12 from the generator bridge 10.

Measurement of the current charging (or discharging) of the bulkcapacitor 82 in the dc link 12 is achieved by adding a small capacitor78 in parallel with the bulk capacitor, measuring the current in thesmall capacitor using a current sensor 80 and re-scaling the measuredcurrent by a factor related to the ratio of the capacitance of the smallcapacitor and the total capacitance of the dc link 12. The sign of thecurrent signal I_CAP is positive when the bulk capacitor 82 is beingcharged and negative when it is being discharged. Since the currentflowing in the bulk capacitor 82 is a switched waveform it is necessaryto integrate the current over a full pulse width modulation (PWM)period.

The inferred signal IDC_NET′ is added to the output of the dc linkvoltage controller 76 at the summing node shown in FIG. 12.

When a supply network voltage dip situation occurs, in the firstinstance the generator bridge 10 is unaware that the network bridge 14is no longer able to export power to the supply network at the previousrate. The surplus current between that being imported from the generator4 and that being exported to the supply network charges the bulkcapacitor 82 and is seen as an increasing signal on the bulk capacitorcharging current derived from the current signal I_CAP. The signalIDC_NET′ is then recalculated and modifies the signal being added to theoutput of the dc link voltage controller 76 hence modifying the actualpower that is being imported from the generator 4 during the supplynetwork voltage dip situation.

In situations where the bulk dc link capacitance of the generator bridge10 and the network bridge 14 are separated by a significant distancethere may be an inter-bridge inductance that causes a resonance betweenthe two decoupled bulk capacitors. In this case, the small capacitor 78may be replaced by a network of two capacitors and an inductor that areselected to achieve the same resonant frequency as the combination ofthe capacitance of the decoupled bulk capacitors and the inter-bridgeinductance. The current is then measured as the current flowing in bothof the small capacitors so that any resonance between them is cancelledout by the measurement process.

The power control for the network bridge 14 will now be described inmore detail with reference to FIG. 13.

IQ_CAPACITY is a signal that relates to the drive rating parameters andthe prevailing network voltage VQ_NET. It will be appreciated that thegenerator bridge 10 and the generator bridge controller 18 together havea finite response time to changes in operating conditions. To takeaccount of this within the network bridge controller 46, the signalIQ_CAPACITY is slew rate limited to produce a limited quadrature axiscurrent demand signal IQ_NET*_LIM that applies only during a supplynetwork voltage dip situation. The slew rate limit is tuned such thatthe limited quadrature axis current demand IQ_NET*_LIM reduces at thesame rate as the power is reduced in the generator bridge 10. The slewrate limit is correctly tuned when the dc link voltage disturbanceoccurring during a supply network voltage dip situation is minimised.

The switch 84 takes the output from a slew-rate limit function whenDIP_DETECT=1 (i.e. when a supply network voltage dip situation isdetermined to exist by the limitation block 86 with reference tospecific network voltage conditions and the parameterisation of thenetwork bridge controller 46). Otherwise, in normal operating situationswhen DIP_DETECT=0 then the quadrature axis current demand signal IQ_NET*is derived from the prevailing network voltage VQ_NET and the ultimatepower limit POWER_LIMIT determined by the drive parameters as shown inFIG. 12. The quadrature axis current demand signal IQ_NET* is limited bya clamp function determined by the power export demand signal P* and theprevailing network voltage VQ_NET. When DIP_DETECT=0 the output of theclamp function is applied to the current controller 58 as the quadratureaxis current demand signal IQ_NET*_LIM.

The limitation block 86 provides a limited direct axis current demandsignal ID_NET*_LIM to the current controller 58 in a supply networkvoltage dip situation. In normal operating situations, the direct axiscurrent demand signal ID_NET* is supplied directly to the currentcontroller 58 as the limited direct axis current demand signalID_NET*_LIM.

A second option for dc link control for the generator bridge 10 of thesecond alternative power controller arrangement will now be describedwith reference to FIGS. 11 and 14.

If the power converter is operating at full capacity, for example, thenthe integral of the PI controller 20 of the dc link voltage controller76 will have a significant value. In the absence of any other controlfeatures, in the event of a network supply voltage dip then a dc linkvoltage error would have to occur to discharge or reset the integralvalue. Such a dc link voltage error would be a positive voltagetransient with a risk of a dc link over-voltage trip occurring due tofinite hardware voltage limitations.

During a supply network voltage dip situation, the rate of change of thevoltage feedback signal VDC_FB indicative of the dc link voltage (asrepresented by dVDC_FB/dt) is significantly larger than would beexperienced during normal operation of the power converter. IfdVDC_FB/dt is greater than a threshold it can be deduced that somethinghas affected the ability of the network bridge 14 to export power and itis probably the case that the network voltage has reduced.

The second option for dc link control shown in FIG. 14 is based on adetermination that if the voltage feedback signal VDC_FB is greater thana first threshold (VDC_FB_THRESHOLD) and dVDC_FB/dt is greater than asecond threshold (dVDC_FB/dt_THRESHOLD) then the integral value in thePI controller 20 is multiplied by a value less than 1 where the value isdetermined by parameterization of the generator bridge controller 18.

If these threshold requirements continue to be exceeded then the same dclink control action will be applied on consecutive PWM scans (i.e. wherea single PWM scan represents one iteration of the control program) suchthat the integral value in the PI controller 20 is sequentially reduced.

The two threshold parameters are based on knowledge of the wind turbinecharacteristics, the maximum expected dVDC_FB/dt during normal operationand the expected dVDC_FB/dt in the event of a grid fault. The maximumexpected dVDC_FB/dt during normal operation can be calculated withknowledge of the dc link capacitance and drive parameters.

Operation of the Alternative Power Converter

One possible operational implementation of the alternative powerconverter topology shown in FIGS. 11 to 14 is as follows. At start-up,the dc link voltage is established by suitable pre-charge circuits (notshown) from the transformer 6 shown in FIG. 11. At this point thesemiconductor power switching devices in the network bridge 14 remaindisabled.

The dc link voltage demand signal VDC₁₃ GEN* applied to the generatorbridge power controller 18 is set to 1100 volts.

Assuming that the wind is blowing and the wind turbine 2 is rotating,when the generator bridge 10 is enabled it will control the direct axiscurrent ID_GEN to achieve the necessary magnetic flux in the generator 4for the prevailing speed conditions, and the quadrature axis currentIQ_GEN will be adjusted under the control of the generator bridge 10 toachieve the objective of a dc link voltage of 1100 volts.

The power export demand signal P* is set to zero and the output of theturbine network voltage controller 88 (and more particularly the PIcontroller 44) is clamped at zero. At this point the semiconductor powerswitching devices in the network bridge 14 are enabled.

In the normal mode of operation where the supply network voltage seen atthe ac terminals of the network bridge 14 is within normal limits thenthe following control action is implemented. The speed sensor signal Nis filtered to provide a first filtered speed signal N′ and a secondfiltered speed N′2. The damping gain KD applied to the second filteredspeed signal N′2 provides damping of shaft resonance in the turbinedrive train. The first filtered speed signal N′ is used as the pointerto a pre-calculated P* versus N′ look-up table 56. The power exportdemand signal P* derived from the look-up table 56 is applied to thepower controller 46 for the network bridge 14. The applied power exportdemand signal P* is divided by the prevailing quadrature axis networkvoltage VQ_NET to obtain a limit signal. This limit signal is appliedvia a clamp function to the quadrature axis current demand signalIQ_NET* to form the quadrature axis current demand signal IQ_NET*_LIM.

In this mode of operation, the quadrature axis current demand signalIQ_NET* is set to a value greater than the maximum value that can bederived from the power export demand signal P* so that the dampingfunction described above remains active.

In the event of a supply network voltage dip, the allocation of ratedoutput power (VA) to the active and reactive axes of the network bridgecontroller 46 will be determined in line with the requirements of thespecific network code for which the wind turbine is parameterised.

Practical Implementations of the Power Converter Topology

The power converter topology arrangements can be implemented as follows.The generator bridge 10 and network bridge 14 can each be implementedusing a MV3000 liquid cooled DELTA inverter module of suitable powerrating. This is an IGBT-based voltage source inverter suitable foroperation on a 690 V ac network with a resulting dc link voltage of 1100V. The generator bridge controller 18 and the network bridge controller46 can each be implemented using a MV3000 DELTA controller. This is amicroprocessor-based electronic controller, the firmware for whichincorporates the functionality necessary to realise the above powercontrol schemes. The microprocessor operates on a fixed time base,sometimes referred to as “scan time”, relating to the pulse widthmodulation (PWM) frequency of the controller. All these products aresupplied by Converteam Ltd of Boughton Road, Rugby, Warwickshire, CV211BU.

Possible Modifications to the Power Converter Topology

The proposed power converters described above can be arranged in asimilar manner if the induction generator 4 is replaced by a permanentmagnet or wound field synchronous generator. In situations where a woundfield synchronous generator is employed, the additional field excitationinput to the generator will typically be used to provide the main fluxwith the direct axis stator current demand signal being set to zero. Forhigh dynamic and/or field weakening situations, the direct axis statorcurrent demand signal may be set to values other than zero to morerapidly ad just the flux in the generator. Typically, the generator willbe a three-phase machine but other phase numbers can be employed. Thepower converter can also be arranged to operate with multi-levelinverters instead of the two-level inverter arrangement described above.

The controller arrangement described above proposes two independentcontrollers that are coordinated by means of control signals being sentfrom the generator bridge controller 18 to the network bridge controller46 and vice versa. It would be equally suitable to integrate thefunctionality of the controllers on to one physical controller.Similarly, the functionality could be spread across more than twocontrollers if this is convenient to the practical implementation of thepower converter.

1. A power converter that can be used to interface a generator thatprovides variable voltage at variable frequency to a supply networkoperating at nominally fixed voltage and nominally fixed frequency, thepower converter comprising: a first active rectifier/inverterelectrically connected to the stator of the generator and including aplurality of semiconductor power switching devices; a second activerectifier/inverter including a plurality of semiconductor powerswitching devices; a dc link connected between the first activerectifier/inverter and the second active rectifier/inverter; a filterconnected between the second active rectifier/inverter and the supplynetwork, the filter including network terminals; a first controller forthe first active rectifier/inverter; and a second controller for thesecond active rectifier/inverter: wherein the first controller uses a dclink voltage demand signal indicative of a desired dc link voltage tocontrol the semiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and wherein the secondcontroller uses a power demand signal indicative of the level of powerto be transferred from the dc link to the supply network through thesecond active rectifier/inverter, and a voltage demand signal indicativeof the voltage to be achieved at the network terminals of the filter tocontrol the semiconductor power switching devices of the second activerectifier/inverter to achieve the desired levels of power and voltagethat correspond to the power and voltage demand signals.
 2. The powerconverter according to claim 1, wherein the first controller uses a fluxdemand signal indicative of a desired level of flux to be achieved inthe generator, converts the flux demand signal to a direct axis currentdemand signal for the first active rectifier/inverter and controls thesemiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired direct axis current for the first active rectifier/inverter.3. The power converter according to claim 2, wherein the firstcontroller converts the flux demand signal to the direct current axisdemand signal with reference to one or more characteristics of thegenerator.
 4. The power converter according to claim 2, wherein thefirst controller compares the dc link voltage demand signal indicativeof a desired dc link voltage to a dc link voltage feedback signal todetermine a quadrature axis current demand signal for the first activerectifier/inverter and controls the semiconductor power switchingdevices of the first active rectifier/inverter to produce statorelectrical quantities that achieve the desired quadrature axis currentfor the first active rectifier/inverter.
 5. The power converteraccording to claim 4, wherein the second controller supplies a controlsignal that varies in accordance with the prevailing supply networkvoltage conditions to the first controller, and wherein the firstcontroller compares the dc link voltage demand signal indicative of adesired dc link voltage to the dc link voltage feedback signal todetermine a dc link current demand signal, limits the dc link currentdemand signal using the control signal from the second controller todetermine a limited dc link current demand signal and uses the limiteddc link current demand signal to determine the quadrature axis currentdemand signal for the first active rectifier/inverter.
 6. The powerconverter according to claim 4, wherein the second controller supplies acontrol signal that varies in accordance with the prevailing supplynetwork voltage conditions and/or the power demand signal to the firstcontroller, and wherein a dc link voltage controller of the firstcontroller compares the dc link voltage demand signal indicative of adesired dc link voltage to the dc link voltage feedback signal toprovide an output signal that is added to the control signal todetermine a dc link current demand signal that is used to determine thequadrature axis current demand signal for the first activerectifier/inverter.
 7. The power converter according to claim 4, whereinthe second controller converts the power demand signal indicative of thelevel of power to be transferred from the dc link to the supply networkthrough the second active rectifier/inverter to a quadrature axiscurrent demand signal for the second active rectifier/inverter andcontrols the semiconductor power switching devices of the second activerectifier/inverter to produce filter/supply network electricalquantities that achieve the desired quadrature axis current for thesecond active rectifier/inverter.
 8. The power converter according toclaim 7, wherein the power demand signal is converted into thequadrature axis current demand signal by dividing the power demandsignal by a signal that is derived from the voltage at the networkterminals of the filter.
 9. The power converter according to claim 7,wherein the power demand signal is converted into the quadrature axiscurrent demand signal by dividing the power demand signal by a filteredversion of the signal that is derived from the voltage at the networkterminals of the filter.
 10. The power converter according to claim 7,wherein the second controller uses a further dc link voltage demandsignal indicative of a desired dc link voltage, compares the further dclink voltage demand signal to the dc link voltage feedback signal todetermine an unlimited quadrature axis current demand signal and limitsthe unlimited quadrature axis current demand signal to a valuedetermined by a limiting signal that is derived from the power demandsignal to determine the quadrature axis current demand signal for thesecond active rectifier/inverter.
 11. The power converter according toclaim 10, wherein the unlimited quadrature axis current demand signal isadded to a quadrature axis current feedforward signal that is derivedfrom: (i) a signal indicative of the generator power, (ii) a voltagefeedback signal measured at the network terminals of the filter and(iii) a gain signal that varies in accordance with the prevailing supplynetwork voltage conditions.
 12. The power converter according to claim11, wherein the signal indicative of the generator power is supplied tothe second controller from the first controller.
 13. The power converteraccording to claim 11, wherein the signal indicative of the generatorpower minus the output of a PI controller of a dc link voltagecontroller of the first controller is supplied to the second controllerand is used by the second controller only during a supply voltage dipsituation.
 14. The power converter according to claim 10, wherein thesecond controller modifies the limiting signal that is derived from thepower demand signal in accordance with the prevailing supply networkvoltage conditions.
 15. The power converter according to claim 1,wherein the dc link includes a capacitor and the power converter furthercomprises a current sensor for measuring the current flowing in thecapacitor and providing an output signal.
 16. The power converteraccording to claim 14, wherein the output signal of the current sensoris subtracted from a signal derived from a signal indicative of thegenerator power to provide a signal that is added to the output of a dclink voltage controller of the first controller to determine a dc linkcurrent demand signal for the first active rectifier/inverter.
 17. Thepower converter according to claim 14, wherein the output signal of thecurrent sensor is subtracted from a signal derived from a signalindicative of the generator power to provide a signal that is filteredand added to the output of a dc link voltage controller of the firstcontroller to determine a dc link current demand signal for the firstactive rectifier/inverter.
 18. The power converter according to claim 1,further comprising a voltage sensor for measuring the dc link voltageand providing a dc link voltage feedback signal and means for measuringthe rate of change of the dc link voltage feedback signal, wherein theintegral value of a PI controller of a dc link voltage controller of thefirst controller is modified by a predetermined factor when the dc linkvoltage feedback signal is greater than a first threshold and the rateof change of the dc link voltage feedback signal is greater than asecond threshold.
 19. The power converter according to claim 1, whereinduring a supply network voltage dip situation, a quadrature axis currentaxis demand signal for the second active rectifier/inverter is derivedfrom a slew rate limited version of a signal derived from the powerlimit rating of the second active rectifier/inverter that is modified asa function of the prevailing supply network voltage conditions.
 20. Thepower converter according to claim 1, wherein the second controllercompares the voltage demand signal indicative of the level of voltage tobe achieved at the network terminals of the filter to a voltage feedbacksignal measured at the network terminals of the filter to determine adirect axis current demand signal for the second activerectifier/inverter and controls the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired direct axiscurrent for the second active rectifier/inverter.
 21. The powerconverter according to claim 20, wherein the second controller modifiesthe direct axis current demand signal in accordance with the prevailingsupply network voltage conditions.
 22. The power converter according toclaim 20, wherein the second controller modifies an error signal arisingfrom the difference between the voltage demand signal indicative of thelevel of voltage to be achieved at the network terminals of the filterand the voltage feedback signal measured at the network terminals of thefilter in accordance with a signal derived from the direct axis currentdemand signal.
 23. The power converter according to claim 1, furthercomprising a speed sensor or speed observer for deriving a speed signalindicative of the speed of the moving part of the generator and whereinthe speed signal is used to derive the power demand signal.
 24. Thepower converter according to claim 23, wherein the power demand signalis derived from a look-up table or mathematical function and the speedsignal forms a pointer to the look-up table or a value for which themathematical function is calculated.
 25. The power converter accordingto claim 23, wherein the speed signal is modified by a filter function.26. The power converter according to claim 25, wherein the power demandsignal is derived from a look-up table or mathematical function and themodified speed signal forms a pointer to the look-up table or a valuefor which the mathematical function is calculated.
 27. The powerconverter according to claim 23, wherein the power demand signal issummed with a signal derived from a filtered version of tile speedsignal.
 28. An arrangement comprising a plurality of power converterseach including: a first active rectifier/inverter electrically connectedto the stator of the generator and including a plurality ofsemiconductor power switching devices; a second activerectifier/inverter including a plurality of semiconductor powerswitching devices; a dc link connected between the first activerectifier/inverter and the second active rectifier/inverter; a filterconnected between the second active rectifier/inverter and the supplynetwork, the filter including network terminals; a first controller forthe first active rectifier/inverter; and a second controller for thesecond active rectifier/inverter; wherein the first controller uses a dclink voltage demand signal indicative of a desired dc link voltage tocontrol the semiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and wherein the secondcontroller uses a power demand signal indicative of the level of powerto be transferred from the dc link to the supply network through thesecond active rectifier/inverter, and a voltage demand signal indicativeof the voltage to be achieved at the network terminals of the filter tocontrol the semiconductor power switching devices of the second activerectifier/inverter to achieve the desired levels of power and voltagethat correspond to the power and voltage demand signals; the powerconverters being connected together in parallel to a supply networkoperating at nominally fixed voltage and nominally fixed frequency by aparallel connection, wherein the voltage demand signal indicative of thevoltage to be achieved at the network terminals of the filter of eachpower converter is derived from a comparison of a top-level voltagedemand signal and a top-level voltage feedback signal that is measuredat the point where the parallel connection is connected to the supplynetwork.
 29. The arrangement according to claim 28, wherein eachindividual power converter includes a step-up transformer electricallyconnected between the associated filter and the parallel connection. 30.The arrangement according to claim 28, further comprising a step-uptransformer electrically connected between the parallel connection andthe supply network.
 31. The arrangement according to claim 30, whereinthe top-level voltage feedback signal is measured at the supply networkside of the step-up transformer electrically connected between theparallel connection and the supply network.
 32. The arrangementaccording to claim 30, wherein the top-level voltage feedback signal ismeasured at the parallel connection side of the step-up transformerelectrically connected between the parallel connection and the supplynetwork.
 33. A wind turbine comprising: a generator having a stator anda rotor; a turbine assembly including at least one blade for rotatingthe rotor of the generator; and a power converter including: a firstactive rectifier/inverter electrically connected to the stator of thegenerator and including a plurality of semiconductor power switchingdevices; a second active rectifier/inverter including a plurality ofsemiconductor power switching devices; a dc link connected between thefirst active rectifier/inverter and the second activerectifier/inverter; a filter connected between the second activerectifier/inverter and the supply network, the filter including networkterminals; a first controller for the first active rectifier/inverter;and a second controller for the second active rectifier/inverter;wherein the first controller uses a dc link voltage demand signalindicative of a desired dc link voltage to control the semiconductorpower switching devices of the first active rectifier/inverter toachieve the desired level of dc link voltage that corresponds to the dclink voltage demand signal; and wherein the second controller uses apower demand signal indicative of the level of power to be transferredfrom the dc link to the supply network through the second activerectifier/inverter, and a voltage demand signal indicative of thevoltage to be achieved at the network terminals of the filter to controlthe semiconductor power switching devices of the second activerectifier/inverter to achieve the desired levels of power and voltagethat correspond to the power and voltage demand signals.
 34. A wind farmcomprising: a supply network operating at nominally fixed voltage andnominally fixed frequency; and a plurality of wind turbines eachincluding: a generator having a stator and a rotor; a turbine assemblyincluding at least one blade for rotating the rotor of the generator;and a power converter including: a first active rectifier/inverterelectrically connected to the stator of the generator and including aplurality of semiconductor power switching devices; a second activerectifier/inverter including a plurality of semiconductor powerswitching devices; a dc link connected between the first activerectifier/inverter and the second active rectifier/inverter; a filterconnected between the second active rectifier/inverter and the supplynetwork, the filter including network terminals; a first controller forthe first active rectifier/inverter; and a second controller for thesecond active rectifier/inverter; wherein the first controller uses a dclink voltage demand signal indicative of a desired dc link voltage tocontrol the semiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and wherein the secondcontroller uses a power demand signal indicative of the level of powerto be transferred from the dc link to the supply network through thesecond active rectifier/inverter, and a voltage demand signal indicativeof the voltage to be achieved at the network terminals of the filter tocontrol the semiconductor power switching devices of the second activerectifier/inverter to achieve the desired levels of power and voltagethat correspond to the power and voltage demand signals; wherein therespective power converters of the plurality of wind turbines areconnected together in parallel to the supply network by a parallelconnection, and wherein the voltage demand signal indicative of thevoltage to be achieved at the network terminals of the filter of eachpower converter is derived from a comparison of a top-level voltagedemand signal and a top-level voltage feedback signal that is measuredat the point where the parallel connection is connected to the supplynetwork.
 35. The wind farm according to claim 34, wherein eachindividual power converter includes a step-up transformer electricallyconnected between the associated filter and the parallel connection. 36.The wind farm according to claim 35, further comprising a step-uptransformer electrically connected between the parallel connection andthe supply network.
 37. The wind farm according to claim 36, wherein thetop-level voltage feedback signal is measured at the supply network sideof the step-up transformer electrically connected between the parallelconnection and the supply network.
 38. The wind farm according to claim36, wherein the top-level voltage feedback signal is measured at theparallel connection side of the step-up transformer electricallyconnected between the parallel connection and the supply network.
 39. Amethod of operating a power converter that can be used to interface agenerator that provides variable voltage at variable frequency to asupply network operating at nominally fixed voltage and nominally fixedfrequency, the power converter comprising: a first activerectifier/inverter electrically connected to the stator of the generatorand including a plurality of semiconductor power switching devices; asecond active rectifier/inverter including a plurality of semiconductorpower switching devices; a dc link connected between the first activerectifier/inverter and the second active rectifier/inverter; a filterconnected between the second active rectifier/inverter and the supplynetwork, the filter including network terminals; a first controller forthe first active rectifier/inverter; and a second controller for thesecond active rectifier/inverter; wherein the method comprises the stepsof: the first controller using a dc link voltage demand signalindicative of a desired dc link voltage to control the semiconductorpower switching devices of the first active rectifier/inverter toachieve the desired level of dc link voltage that corresponds to the dclink voltage demand signal; and the second controller using a powerdemand signal indicative of the level of power to be transferred fromthe dc link to the supply network through the second activerectifier/inverter, and a voltage demand signal indicative of thevoltage to be achieved at the network terminals of the filter to controlthe semiconductor power switching devices of the second activerectifier/inverter to achieve the desired levels of power and voltagethat correspond to the power and voltage demand signals.
 40. The methodaccording to claim 39, further comprising the step of the secondcontroller using a measure of the supply network voltage to determinelimits on the power that can be exported from the second activerectifier/inverter when the supply network voltage deviates from itsnominal condition.
 41. The method according to claim 39, furthercomprising the step of the second controller using a measure of thesupply network voltage to determine the level of current that is to beprovided from the second active rectifier/inverter to provide voltagesupport to the supply network when the supply network voltage deviatesfrom its nominal condition.
 42. The method according to claim 39,further comprising the step of the first controller using a flux demandsignal indicative of a desired level of flux to be achieved in thegenerator, converting the flux demand signal to a direct axis currentdemand signal or the first active rectifier/inverter and controlling thesemiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired direct axis current for the first active rectifier/inverter.43. The method according to claim 42, wherein the step of converting theflux demand signal to the direct current axis demand signal is carriedout with reference to one or more characteristics of the generator. 44.The method according to claim 39, further comprising the step of thefirst controller comparing the dc link voltage demand signal indicativeof a desired dc link voltage to a dc link voltage feedback signal todetermine a quadrature axis current demand signal for the first activerectifier/inverter and controlling the semiconductor power switchingdevices of the first active rectifier/inverter to produce statorelectrical quantities that achieve the desired quadrature axis currentfor the first active rectifier/inverter.
 45. The method according toclaim 44, further comprising the steps of: the second controllersupplying a control signal that varies in accordance with the prevailingsupply network voltage conditions to the first controller during asupply network voltage dip situation; and the first controller comparingthe dc link voltage demand signal indicative of a desired dc linkvoltage to the dc link voltage feedback signal to determine a dc linkcurrent demand signal, limiting the dc link current demand signal usingthe control signal from the second controller to determine a limited dclink current demand signal and using the limited dc link current demandsignal to determine the quadrature axis current demand signal for thefirst active rectifier/inverter so that no power is drawn from thesupply network during the supply network voltage dip situation.
 46. Themethod according to claim 44, further comprising the steps of the secondcontroller supplying a control signal that varies in accordance with theprevailing supply network voltage conditions and/or the power demandsignal to the first controller and a dc link voltage controller of thefirst controller comparing the dc link voltage demand signal indicativeof a desired dc link voltage to the dc link voltage feedback signal toprovide an output signal that is added to the control signal todetermine a dc link current demand signal that is used to determine thequadrature axis current demand signal for the first activerectifier/inverter.
 47. The method according to claim 44, furthercomprising the step of the second controller converting the power demandsignal indicative of the level of power to be transferred from the dclink to the supply network through the second active rectifier/inverterto a quadrature axis current demand signal for the second activerectifier/inverter and controlling the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired quadrature axiscurrent for the second active rectifier/inverter.
 48. The methodaccording to claim 47, wherein the step of converting the power demandsignal to the quadrature axis current demand signal is carried out bydividing the power demand signal by a signal that is derived from thevoltage at the network terminals of the filter.
 49. The method accordingto claim 48, wherein the step of converting the power demand signal intothe quadrature axis current demand signal is carried out by dividing thepower demand signal by a filtered version of the signal that is derivedfrom the voltage at the network terminals of the filter.
 50. The methodaccording to claim 47, further comprising the step of the secondcontroller using a further dc link voltage demand signal indicative of adesired dc link voltage, comparing the further dc link voltage demandsignal to the dc link voltage feedback signal to determine an unlimitedquadrature axis current demand signal and limiting the unlimitedquadrature axis current demand signal to a value determined by alimiting signal that is derived from the power demand signal todetermine the quadrature axis current demand signal for the secondactive rectifier/inverter during start-up and the normal operatingcondition of the power converter.
 51. The method according to claim 50,further comprising the step of adding the unlimited quadrature axiscurrent demand signal to a quadrature axis current feedforward signalthat is derived from: (i) a signal indicative of the generator power,(ii) a voltage feedback signal measured at the network terminals of thefilter and (iii) a gain signal that varies in accordance with theprevailing supply network voltage conditions.
 52. The method accordingto claim 51, wherein the signal indicative of the generator power issupplied to the second controller from the first controller.
 53. Themethod according to claim 51, wherein the signal indicative of thegenerator power minus the output of a PI controller of a dc link voltagecontroller of the first controller is supplied to the second controllerand is used by the second controller only during a supply voltage dipsituation.
 54. The method according to claim 50, further comprising thestep of the second controller modifying the limiting signal that isderived from the power demand signal in accordance with the prevailingsupply network voltage conditions in a supply network voltage dipsituation.
 55. The method according to claim 39, wherein the dc linkincludes a capacitor and the power converter further comprises a currentsensor for measuring the current flowing in the capacitor and providingan output signal, the method further comprising the steps of subtractingthe output signal of the current sensor from a signal derived from asignal indicative of the generator power to provide a signal that isadded to the output of a dc link voltage controller of the firstcontroller to determine a dc link current demand signal for the firstactive rectifier/inverter.
 56. The method according to claim 39, whereinthe dc link includes a capacitor and the power converter furthercomprises a current sensor for measuring the current flowing in thecapacitor and providing an output signal, the method further comprisingthe steps of subtracting the output signal of the current sensor from asignal derived from a signal indicative of the generator power toprovide a signal that is filtered and added to the output of a dc linkvoltage controller of the first controller to determine a dc linkcurrent demand signal for the first active rectifier/inverter.
 57. Themethod according to claim 39, wherein the power converter furthercomprises a voltage sensor for measuring the dc link voltage andproviding a dc link voltage feedback signal and means for measuring therate of change of the dc link voltage feedback signal, the methodfurther comprising the steps of modifying the integral value of a PIcontroller of a dc link voltage controller of the first controller by apredetermined factor when the dc link voltage feedback signal is greaterthan a first threshold and the rate of change of the dc link voltagefeedback signal is greater than a second threshold.
 58. The methodaccording to claim 39, further comprising the step of deriving aquadrature axis current axis demand signal for the second activerectifier/inverter from a slew rate limited version of a signal derivedfrom the power limit rating of the second active rectifier/inverter thatis modified as a function of the prevailing supply network voltageconditions in a supply network voltage dip situation.
 59. The methodaccording to claim 39, further comprising the step of the secondcontroller comparing the voltage demand signal indicative of the levelof voltage to be achieved at the network terminals of the filter to avoltage feedback signal measured at the network terminals of the filterto determine a direct axis current demand signal for the second activerectifier/inverter and controlling the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired direct axiscurrent for the second active rectifier/inverter.
 60. The methodaccording to claim 59, further comprising the step of the secondcontroller modifying the direct axis current demand signal in accordancewith the prevailing supply network voltage conditions.
 61. The methodaccording to claim 59, further comprising the step of the secondcontroller modifying an error signal arising from the difference betweenthe voltage demand signal indicative of the level of voltage to beachieved at the network terminals of the filter and the voltage feedbacksignal measured at the network terminals of the filter in accordancewith a signal derived from the direct axis current demand signal. 62.The method according to claim 39, further comprising the step ofderiving a speed signal indicative of the speed of the moving part ofthe generator and using the speed signal to derive the power demandsignal.
 63. The method according to claim 62, further comprising thestep of deriving the power demand signal from a look-up table ormathematical function where the speed signal forms a pointer to thelook-up table or a value for which the mathematical function iscalculated.
 64. The method according to claim 62, further comprising thestep of modifying the speed signal by a filter function.
 65. The methodaccording to claim 64, further comprising the step of deriving the powerdemand signal from a look-up table or mathematical function where themodified speed signal forms a pointer to the look-up table or a valuefor which the mathematical function is calculated.
 66. The methodaccording to claim 62, further comprising the step of summing the powerdemand signal with a signal derived from a filtered version of the speedsignal.
 67. A method of operating a plurality of power converters eachincluding: a first active rectifier/inverter electrically connected tothe stator of the generator and including a plurality of semiconductorpower switching devices; a second active rectifier/inverter including aplurality of semiconductor power switching devices; a dc link connectedbetween the first active rectifier/inverter and the second activerectifier/inverter; a filter connected between the second activerectifier/inverter and the supply network, the filter including networkterminals; a first controller for the first active rectifier/inverter;and a second controller for the second active rectifier/inverter;wherein the first controller uses a dc link voltage demand signalindicative of a desired dc link voltage to control the semiconductorpower switching devices of the first active rectifier/inverter toachieve the desired level of dc link voltage that corresponds to the dclink voltage demand signal; and wherein the second controller uses apower demand signal indicative of the level of power to be transferredfrom the dc link to the supply network through the second activerectifier/inverter, and a voltage demand signal indicative of thevoltage to be achieved at the network terminals of the filter to controlthe semiconductor power switching devices of the second activerectifier/inverter to achieve the desired levels of power and voltagethat correspond to the power and voltage demand signals; the powerconverters being connected together in parallel to a supply networkoperating at nominally fixed voltage and nominally fixed frequency by aparallel connection, the method comprising the step of deriving thevoltage demand signal indicative of the voltage to be achieved at thenetwork terminals of the filter of each power converter from acomparison of a top-level voltage demand signal and a top-level voltagefeedback signal that is measured at the point where the parallelconnection is connected to the supply network.
 68. The method accordingto claim 67, further comprising the step of measuring the top-levelvoltage feedback signal at the supply network side of the step-uptransformer electrically connected between the parallel connection andthe supply network.
 69. The method according to claim 67, furthercomprising the step of measuring the top-level voltage feedback signalat the parallel connection side of the step-up transformer electricallyconnected between the parallel connection and the supply network.
 70. Amethod of operating a wind turbine comprising a generator that providesvariable voltage at variable frequency and has a stator and a rotor, aturbine assembly including at least one blade for rotating the rotor ofthe generator, and a power converter that interfaces the generator to asupply network operating at nominally fixed voltage and nominally fixedfrequency, the power converter including: a first activerectifier/inverter electrically connected to the stator of the generatorand including a plurality of semiconductor power switching devices; asecond active rectifier/inverter including a plurality of semiconductorpower switching devices; a dc link connected between the first activerectifier/inverter and the second active rectifier/inverter; a filterconnected between the second active rectifier/inverter and the supplynetwork, the filter including network terminals; a first controller forthe first active rectifier/inverter; and a second controller for thesecond active rectifier/inverter; wherein in response to a change inwind speed the method comprises the steps of: controlling the secondactive rectifier/inverter to change the level of power exported out ofthe dc link such that the dc link voltage changes from a desired level;and controlling the first active rectifier/inverter to import sufficientcurrent into the dc link through the generator bridge from the generatorto restore the dc link voltage to the desired level.